It is proposed to analyze the mechanism of initiation of DNA replication in three closely related small DNA viruses phi X174, G4 and ST-1) that replicate their DNA using different host cell proteins. The structure of the initiation site in both strands of the DNA duplex will be investigated, together with the proteins that each strand requires for initiation of synthesis of its complementary DNA strand. The structure and relationship of the two sites will be correlated with the model of DNA replication that each virus uses and the structure of the initiation sites will be changed by in vivo and in vitro mutagenesis in order to analyze the requirements of structure on their function. These studies will indicate whether the initiation of synthesis of each strand of the DNA duplex is independent. The overall objective is to learn enough about the sites of initiation of DNA synthesis to be able to specifically inactivate different origins of DNA replication in the same cell and to be able to design a rational method of specifically inactivating invading viruses or resident oncogenic viruses.